tmux - handy and versatile when using the command line and running several programs at once. Also, very handy for headless computers so that you can disconnect and leave software running, then reconnect later right where you were.

rtorrent - for downloading, but also for seeding Debian installation discs. I like to keep at least one Raspberry Pi on each lan I use seeding Debian. And for utility use.

mc - good old midnight commander for file managing.

htop - to keep track of processes.

vim - word processing.

ssh - for operating computers remotely. Now that Bash can be installed in Windows, ssh allows me to use my Debian desktop at work from the company computers easily.

rsync - backing up files, or simply keeping files synchronized over several computers.

rangerI'm absolutely loving this file manager. Took a bit of getting used to, but I can navigate directories, understand deep file trees, and just generally find things that I'm looking for far faster than I've ever been able to do in a graphical file manager.

vimThis doesn't really need much explaination, I suppose, but I love it and use it every day.

unisonIf you're looking for a local dropbox-like replacement that runs anywhere, check this out. It's awesome.

mpsytMost portable youtube browser. Great for things like the raspberry pi which don't handle inefficient video streaming in a web browser. Be sure to install with pip to get the latest version, and configure it to search more than just music. As a plus, you can easily download videos in addition to streaming.

youtube-dlI generally have this setup to download the most recent 3 videos from playlists of channels I watch often, either audio-only or low quality video, because I like watching/listening to things locally/offline and video is huge and ssds are small.

apt-cdromOk, I only use this to initially load repo dvd images into apt, but it's really awesome to be able to install packages offline. This is one of the main reasons I use debian over other distros. I can tinker with my system and install things even when I'm riding a plane, train, or bus, or can't find wifi, or even when I'm hiking/camping, or when internet goes down.